This invention relates to a vacuum-tight assembly where a metal is joined to a ceramic article such as a metal vapor arc lamp discharge tube.
Ceramic compositions are used widely in high temperature environments where good chemical and mechanical stability are required. However, the great majority of ceramic compositions are opaque to visible light and therefore cannot be used in a high temperature environment where a transparent window, lamp envelope or the like are required. For example, high pressure sodium lamps are now widely used because they have a higher light-output efficiency as compared to incandescent lamps generally and they have an efficiency second only to the low pressure sodium lamp. The discharge tubes for the sodium metal vapor arc lamps are constructed from a polycrystalline alumina ceramic or from single crystal sapphire. Inside the presently utilized alumina or sapphire discharge tubes, an amalgam of sodium and mercury metals and some inert gases such as xenon are utilized to generate light. Surrounding the arc tube is an evacuated outer lamp envelope made of glass. A minimum temperature of several hundred degrees is required to vaporize the sodium and an electric arc is utilized to heat the central section of the arc tube to about 1200.degree. C. The material forming the arc tube, in addition to being transparent or translucent, must also be stable for a considerable time period against reaction with the sodium and mercury components within the tube at its operating temperature.
While the polycrystalline alumina ceramics and single crystal sapphire have the requisite degree of transparency for use in such discharge tubes, their use was proven undesirable since they tend to dissociate and vaporize as aluminum metal from the outside of the arc tube, thereby darkening the inner surface of the glass envelope of the lamp. The rate at which this takes place determines the useful lamp life and depends both on the tube temperature and the oxygen pressure within the outer envelope. This limitation on lamp life can be overcome therefore by placing a lower limit on oxygen pressure or an upper limit on arc tube operating temperature. However, neither approach is desirable. In presently utilized arc lamps, a niobium seal is utilized to seal the sodium vapor within the arc tube. However, niobium is permeable to oxygen and any attempt to permit oxygen in the outer envelope eventually leads to sodium loss within the arc tube by oxidation to sodium aluminate. On the other hand, placing an upper limit on arc tube temperature results in an undesirable reduction of light-output efficiency.
Accordingly, it would be highly desirable to provide ceramic compositions which are transparent or translucent to visible light, and which are stable under the high temperature operating conditions of sodium vapor arc lamps. With the advent of such improved ceramic compositions, it is necessary to provide suitable sealing means for maintaining a vacuum tight discharge tube so that the fill components do not leak into the surrounding evacuated lamp envelope. Such sealing means must be chemically, mechanically and thermodynamically stable for long periods under the high temperature conditions of operation of such lamps. Accordingly, it is necessary that the sealing means be essentially unreactive with the vapor utilized in the lamp and that it have a coefficient of thermal expansion as most nearly similar to the ceramic composition as possible.